Fiber reinforced plastic, typically referred to as FRP, may find increasing usage in the automotive industry, despite its higher cost, because of its high strength to weight ratios. An example is windshield wiper arms, which are traditionally metal components. As windshields are sloped back ever farther for aerodynamic efficiency, their wiper arms grow ever longer and heavier. The stress created by the extra weight at wiper reversal could require heavier and more expensive wiper motors and linkages, making a lighter FRP arm potentially cost effective. One problem with substituting FRP for metal in any automotive component is the fact that it is difficult or impossible to form it into shapes that are convoluted or discontinuous. Thus, it may serve well as a drive shaft, which is an elongated tube of constant cross section, but not as a transmission case, with its labyrinthine internal passages.
Another limitation is that many automotive components must be attached directly to another metal component at some point, which may require that the FRP component be provided with a localized metal fastening member. For example, an FRP drive shaft must have a metal connector at each end for attachment to the rest of the drive line. It is difficult to successfully and securely mate FRP directly to metal, especially when the attachment point will be subject to heavy loading and stress. Many patents are directed just to the problem of joining metal end pieces to FRP drive shafts, most of which involve various adhesives, rivets, splines or combinations thereof.
The designer of an FRP wiper arm would face both problems noted above. The main body of a wiper arm is basically a rod or beam with a fairly constant cross section and smooth exterior surface, presenting no particular protrusions or discontinuities. This is a basic shape that would lend itself well to FRP manufacture. A matrix of full length reinforcing glass fibers soaked with a conventional thermosetting resin is laid out in a mold with the desired beam shape, and then heat cured. However, each end of the beam must be connected to other structures, one to the wiper blade and one to the knurled wiper drive post. The end connection to the wiper post, especially, requires a complex shape and is subject to high stresses that are much better served by a metal to metal connection. Die casting a metal drive post connector directly to the end of an FRP arm would be preferable, in terms of time, cost and strength, to attaching a separate connector by adhesive or mechanical means. However, the thermoset resin that binds the fibers together decomposes badly at the melting temperatures of suitable metals, such as aluminum alloy. Tests that subjected FRP to molten metal for times comparable to the cycle times involved in standard die casting operations found such severe thermal decomposition of the resin as to conclude that the process would not be feasible.